1. Field of the Invention
The present invention relates to a piezoelectric transformer used for various types of power supplies. Particularly, it relates to a small-sized, thin-formed, and light-weight power supply capable of a large-capacity output.
2. Background Art
Conventionally, a wire-wound type electromagnetic transformer has been used as a transformer device used in electronic instruments such as various types of household electrical appliances or audio visual systems. Since such electromagnetic transformer is structured by winding a lead wire around a magnetic core, it is necessary to increase the number of lead wires to be wound for realizing a high transformation ratio. In addition, it has been very difficult to realize a small-sized, thin-formed electromagnetic transformer due to the need to assure a withstand voltage.
In contrast to such electromagnetic wound transformer, a piezoelectric transformer has been proposed (see Patent Document 1). This piezoelectric transformer has a principle of operation utterly different from that of the conventional electromagnetic transformer. FIG. 5 shows a perspective view of a structure of a single plate type piezoelectric transformer, which is what is called a Rosen-type piezoelectric transformer. The structure of such piezoelectric transformer will be hereafter described with reference to the drawing.
The portion provided with planar electrodes 35 on the top and bottom surfaces of a piezoelectric body refers to a driving portion 4, and it is polarized in the thickness direction thereof. Further, the portion sandwiched between an end face electrode 36 provided on an end portion of the piezoelectric body and the driving portion 4 is a power-generating portion 8, and it is polarized in the length direction thereof. In the piezoelectric transformer with such structure, a support 40 is attached to a node point during resonance of longitudinal vibration with respect to the length direction, and it is fixed to a transformer substrate (not shown). In this state, by applying an alternating voltage having a resonance frequency of the longitudinal vibration with respect to the length direction of the piezoelectric body between an external input electrical terminal 37 connected to the upper planar electrode and an external common electrical terminal 38 connected to the lower planar electrode, a voltage is generated between the external common electrical terminal 38 and an external output electrical terminal 39 connected to the end face output electrode 36. However, the piezoelectric transformer with such single plate structure as shown in FIG. 5 has an output current of on the order of several mA. Thus, it is not suitable for a power supply such as an AD adapter that requires an output current of on the order of several A.
Use of a piezoelectric transformer having a multilayer structure has been a means of making a piezoelectric transformer capable of large-capacity output. For example, Patent Documents 2 and 3 disclose such piezoelectric transformer having a multilayer structure. In the technology disclosed in Patent Document 2, a step-down voltage is generated by generating vibration in the length direction of a power-generating portion in conjunction with vibration in the length direction of a driving portion. An alternating voltage having a primary resonance frequency with respect to the length direction is applied to the driving portion via an external input electrical terminal, and a step-down voltage is generated at the power-generating portion via an external output electrical terminal. The technology disclosed in Patent Document 3 is characterized in that an alternating voltage having a tertiary resonance frequency is applied.
Patent Document 1: U.S. Pat. No. 2,830,274
Patent Document 2: JP Patent Publication (Kokai) No. 8-153914 A (1996)
Patent Document 3: JP Patent Publication (Kokai) No. 5-235432 A (1993)